This invention relates to a microporous shaped article having fine network open-cellular pores.
Microporous shaped articles of various shapes having fine network open-cellular pores have been known. For example, microporous films have been widely used as a battery separator or a diaphragm of a capacitor, and hollow filaments have been widely used an air filter, a gas separation membrane, and a membrane for blood purification in a therapeutic field, and as a dialysis membrane. Typically, for production, a cellulose ester or polyester resin is dissolved in a solvent, and the solution is extruded from a spinneret of a double-walled tube and guided into a coagulating liquid. Another known method of production comprises mixing a filler composed of an inorganic compound typified by calcium carbonate and silica with a polyolefin, shaping the mixture in the molten state, and then stretching the shaped product (Japanese Patent Publication No. 49405/1986). The first-mentioned method is easy and can give a shaped article having excellent permeation property. But the shaped article lacks chemical resistance and has inferior mechanical strength and particularly inferior break strength and elongation. In incorporating the hollow filament membranes in a helical or U-shape into a module, the hollow filaments tend to break and lose the excellent mechanical properties of the material. The latter method can give shaped articles form which the above defects have been removed to some extent. But the chemical resistance of the shaped article by the latter method is not sufficient. Furthermore, its pore size cannot be controlled, and its porosity cannot be increased above a certain limit. Thus, it is difficult to have the shaped article exhibit sufficient separation and permeation properties.
It is an object of this invention therefore to provide a microporous shaped article having excellent mechanical strength and chemical resistance in which the pore diameter is controlled to a uniform value.
Another object of this invention is to provide a technique of making a microporous shaped article having excellent separating and permeating abilities easily at low cost.
Other objects of this invention will become apparent from the following description.
According to this invention, there is provided a microporous shaped article comprising a polyolefin and synthetic resin particles dispersed therein and having a softening temperature or a decomposition temperature higher than the shaping temperature of the polyolefin, said article having a network structure composed of open-cellular pores with a maximum pore diameter of not more than 5 micrometers, having a porosity of 20 to 90% and being molecularly oriented by stretching.
The microporous shaped article of this invention is composed mainly of a polyolefin and synthetic resin particles dispersed in the polyolefin.
The polyolefin is not particularly limited, and any conventional polyolefins can be used. Typical examples of polyolefins that can be used particularly suitably include homopolymers of alpha-olefins such as polyethylene, polypropylene, polybutene-1 and polymethylpentene, copolymers of alpha-olefins and other copolymerizable monomers, and mixtures of these. In view of the thermal stability of the microporous article of the invention as well as shapeability, a propylene homopolymer, copolymers of propylene with other copolymerizable monomers and mixtures of these are preferred.
The copolymers of alpha-olefins generally contain at least 90% by weight of an alpha-olefin, particularly propylene and not more than 10% by weight of another copolymerizable monomer. The copolymerizable monomers are not particularly limited, and any known monomers may be used. Generally, alpha-olefins having 2 to 8 carbon atoms, especially ethylene and butene, are preferred.
The synthetic resin particles used in this invention act to induce peeling in the interface with the polyolefin and form open-cellular pores. Accordingly, the synthetic resin particles used have a softening temperature or a decomposition temperature higher, preferably at least 10.degree. C., especially at least 100.degree. C. higher, than the shaping temperature of the polyolefin. Preferably, the synthetic resin particles, when mixed with the polyolefin, are not agglomerated but are dispersed uniformly.
The synthetic resin particles used in this invention may be particles of any known synthetic thermosetting and thermoplastic resins which perform the above function. Above all, particles of thermosetting resin having a crosslinked structure are preferably used. If the softening or decomposition temperature of the synthetic resin particles is lower than the shaping temperature of the polyolefin, the synthetic resin particles may be softened or decomposed to evolve gases during the formation of a shaped article, and a microporous shaped article cannot be obtained.
Specific examples of the synthetic resin particles preferably used in this invention include polyamides such as 6-nylon and 6,6-nylon; fluorine-containing resins such as polytetrafluoroethylene and tetrafluoroethylene-hexafluoropropylene copolymer; polyimides, silicone resins; phenolic resins; benzoguanamine resins; and crosslinked copolymers of styrene, acrylic acid, methacrylic acid, methyl acrylate or methyl methacrylate and a divinyl compound such as divinylbenzene. Particularly a crosslinked polymer is preferably used. Above all, the silicone resins are most preferably used because the interface between the polyolefin and the synthetic resin particles has good peelability, and by stretching, the shaped article can be easily rendered porous.
The synthetic resin particles should have an average particle diameter of 0.01 to 5 micrometers. If the average particle diameter of the synthetic resin particles falls outside the above range, the synthetic resin particles are difficult to disperse in the polyolefin, or they have too large a maximum pore diameter so that the resulting product cannot be used for such applications as liquid separation, reverse osmosis, ultrafiltration and gas separation. To obtain porous shaped articles that can be preferably accepted in such applications, the synthetic resin particles preferably have an average particle diameter of 0.03 to 3 micrometers. Preferably, the synthetic resin particles have as narrow a particle size distribution as possible because with narrower particle size distribution, a more uniform pore size can be obtained. Generally, if the particle size distribution is expressed by S.sup.2, namely the average of the square of the difference from the average ##EQU1## S.sup.2 is preferably not more than 1.5, especially not more than 0.1. The shape of the synthetic resin particles may be any. Usually, they are preferably in the form of a spherical or elliptical particle having a long-to-short diameter ratio of from 1 to 2 because pores having a uniform diameter can be obtained. The above ratio is especially preferably from 1 to 1.5.
Since the synthetic resin particles used in this invention are synthesized industrially, the above-mentioned uniform particles can be obtained. This uniformity brings about the following advantage over non-uniform particles of an inorganic compound obtained, for example, by crushing fine particles of the compound. Generally, when an inorganic compound is used as a filler, the largest amount of the filler to be added is about 40% by volume based on the polyolefin. Unexpectedly, however, when synthetic resin particles are used as the filler, 55% by volume or even more of them can be filled into the polyolefin. As a result, the porosity of the microporous shaped article obtained can reach even 90%. Moreover, since they have a uniform particle diameter, the dispersion of these particles in the resin is good, and the pores in the microporous shaped article can be controlled nearly to a uniform pore diameter. Another important advantage is that while the inorganic compound has insufficient chemical resistance, the synthetic resin particles have high chemical resistance and are not limited in use for lack of chemical resistance.
The blending ratio between the polyolefin (a) and the synthetic resin particles (b) constituting the microporous shaped article may be determined properly depending upon the properties required of the final microporous shaped article. Most broadly, the proportion of (a) is generally 20 to 80% by weight, preferably 30 to 70% by weight, and the proportion of the synthetic resin particles (b) is generally 80 to 20% by weight, preferably 70 to 30% by weight. The above proportions of the component (a) and the component (b) are important for maintaining the properties of the microporous shaped article within the above-specified ranges and producing the microporous shaped article industrially advantageously. If the proportion of component (b) is lower than the above-specified lower limit, the formation of pores in the resulting microporous shaped article is not sufficient, and the desired porosity sometimes cannot be obtained. If, on the other hand, the proportion of component (b) is higher than the specified upper limit, the shapability of the starting composition tends to become poor. Consequently, sometimes, stretching tends to be unable to be carried out sufficiently, and a sufficient porosity tends to be unable to imparted to the shaped article.
Since the particle diameter of the microporous shaped article is affected by the particle diameter of the synthetic resin particles, its required particle diameter can be obtained by controlling the particle diameter of the synthetic resin particles obtained. Generally, when synthetic resin particles having an average particle diameter of 0.01 to 5 micrometers are used, the resulting microporous shaped article has a maximum pore diameter of not more than 5 micrometers and an average particle diameter of generally 0.02 to 3 micrometers. Because the synthetic resin particles are uniformly dispersed in the polyolefin, the pores of the microporous shaped article are of a network structure composed of open-cellular pores. The porosity of the microporous shaped article is determined depending upon the blending proportion of the synthetic resin particles, the stretch ratio, etc. and can generally be selected from the range of 20 to 90%, preferably the range of 35 to 80%.
Other properties of the microporous shaped articles arc the same irrespective of their shapes, if the production conditions are the same. However, since the shape and the mode of use of the microporous shaped article frequently differ according to usages, some typical properties and shapes are exemplified below.